1. Field of the Invention
The present invention relates to a projection lens assembly and a projection display apparatus for projecting optical images, formed on respective light valves, onto a screen on an enlarged scale.
2. Description of the Prior Art
Various methods have been practiced to project a large picture on a screen. One of them which appears to be relevant to the present invention includes forming an optical image on a light valve according to a video signal applied thereto, irradiating the optical image to produce images of light and passing the images of light through a projection lens assembly so as to be projected onto a screen on an enlarged scale. For the light valve, a recent trend is to employ a liquid crystal panel. Hence, the projection display system utilizing the liquid crystal panel as the light valve is well known in the art.
For example, Japanese Laid-open Patent Publication No. 62-133424, published Jun. 16, 1987, discloses a projection display apparatus of a separate type comprising a projector assembly and a screen. The projector assembly comprises three dichroic mirrors for separating rays of light of visible wavelength, radiated from a light source, into three primary color light components, respectively, three dichroic mirrors for combining the three color light components and three liquid crystal panels for displaying images in three colors, respectively. Also, U.S. Pat. No. 5,042,929 (corresponding to the Japanese Laid-open Patent Publication No. 2-250015, published in 1992) discloses a projection display apparatus of an integrated type comprising a cabinet having a back-lighting projection screen and a projector accommodated within the cabinet together with an optical system.
For reproducing projected images of high quality, the liquid crystal panel is known to comprise twisted nematic liquid crystal as a liquid crystal material and a plurality of pixels each constituted by a switching element in the form of a thin-film transistor. For driving the switching elements, an active matrix drive circuit is generally employed. Specifically, in a color picture reproduction, three liquid crystal panels are employed one for each of the three primary colors, i.e., red, green and blue.
The separate and integrated types of the prior art picture projection systems are shown in FIGS. 32 and 33, respectively, and reference will now be made thereto.
Referring first to FIG. 32 showing the separate type, rays of light emanating from a light source 11 pass through a color separating optical system, including dichroic mirrors 12 and 13 and a planar mirror 14, for separating the incident rays of light into color light components of three primary colors, that is, red, green and blue light components. These light components are, after having passed through associated field lenses 15, 16 and 17, projected onto the respective liquid crystal panels 18, 19 and 20. In response to a video signal applied to the liquid crystal panels 18, 19 and 20, the latter are driven to form respective optical images thereon each as a function of a change in light transmittance. Discrete images of light passing through the respective liquid crystal panels 18, 19 and 20 are then passed through a color combining optical system, including dichroic mirrors 21 and 22 and a planar mirror 23, to produce composite images of light which are subsequently projected through a projection lens assembly 24 onto a separate screen on an enlarged scale.
Referring now to FIG. 33 showing a projection optical system employed in the prior art integrated type, rays of light emanating from a light source 31 pass through a color separating optical system, including dichroic mirrors 32 and 33 and a planar mirror 34, for separating the incident rays of light into color light components of three primary colors, that is, red, green and blue light components. These light components are, after having passed through associated liquid crystal panels 35, 36 and 37, transmitted to respective projection lens assemblies 38, 39 and 40. When at this time the liquid crystal panels 35, 36 and 37 are driven in response to a picture signal applied thereto, respective optical images are formed on those liquid crystal panels 35, 36 and 37 each as a function of a change in light transmittance. Discrete images of light having passed through the respective liquid crystal panels 35, 36 and 37 are projected onto a common screen, shown by 42 in FIG. 34, by means of the projection lens assemblies 38, 39 and 40 so as to combine together to produce a composite color picture on the screen 42 on an enlarged scale.
According to the integrated type, in order for the discrete images of light having passed through the respective liquid crystal panels 35, 36 and 37 to be properly aligned with each other on the screen to produce the composite color picture, the projection lens assemblies 38, 39 and 40 have their respective optical axes lying parallel to each other while the liquid crystal panels 35 and 37 on respective sides of the liquid crystal panel 36 which occupies an intermediate position have their respective centers slightly offset from the associated optical axes of the projection lens assemblies 38 and 40.
The optical system shown in FIG. 33 is housed within a cabinet 41 of a generally rectangular box-like configuration as shown in FIG. 34 together with the screen 42. As shown in FIG. 34, the cabinet 41 has a portion of the front panel defined by the screen 42 and the projector 43 installed at a rear bottom of the interior of the cabinet 41. In order for the discrete images of light having passed through the respective projection lens assemblies 38, 39 and 40 to be projected onto the screen 42, two planar mirrors 44 and 45 are disposed within the cabinet 41 along an optical path from the projector 43 to the screen 42. The planar mirror 44 is positioned generally beneath the screen 42 so as to confront the projector 43 while the planar mirror 45 is positioned rearwardly of the screen 42 and generally above the projector 43, so that the discrete images of light travel in a generally zig-zag fashion from the projector 43 towards the screen 42.
The disposition of the component parts of the optical system shown in FIG. 34 is effective to minimize the distance over which the discrete images of light travel from the projector 43 to the screen 42, making it possible to minimize the size of the cabinet 41 and, hence, the projection display apparatus as a whole. As a matter of course, the screen 42 is of a sandwich structure including a Fresnel lens and a lenticular plate as is well known to those skilled in the art.
Any of the prior art projection display apparatuses shown in FIGS. 33 and 34 employs the three projection lens assemblies and, therefore, involves problems associated with color shift (the phenomenon in which the color tone of the projected color picture varies with a change in a viewing angle) and color non-uniformity. The color shift may be reduced to a certain extent by the use of the lenticular plate having a plurality of lenticular lenslets on its opposite surfaces, but a complete elimination of the color shift is impossible. On the other hand, the color non-uniformity may be compensated for to a certain extent by the use of a suitably designed electronic circuit, but a complete compensation is difficult to achieve.
Both the color shift and the color non-uniformity result from the use of the three projection lens assemblies and, in order to substantially eliminate those problems, a single projection lens assembly should be used together with the color combining optical system utilizing the dichroic mirrors for combining the discrete images of light to provide the composite color picture. In such case, the dichroic mirrors may be arranged so as to be either parallel to each other such as shown in FIG. 32 or in a generally X-shaped layout such as disclosed in Japanese Laid-open Patent Publication No. 63-116123 published in 1988.
In the parallel arrangement of the dichroic mirrors referred to above, the length of the irradiating optical path as measured from the light source to each of the liquid crystal panels is equal for each of the three primary colors, but in the X-shaped arrangement of the dichroic mirrors referred to above, the length of one of the irradiating optical paths associated with one of the three primary colors is greater than that of any one of the irradiating optical paths associated with the remaining two of the three primary colors. Since the presence of a difference in length of the irradiating optical paths for the three primary colors tends to result in a color non-uniformity, the prior art system shown in FIG. 32 appears to be more feasible.
Referring again to FIGS. 32 and 33, the prior art projection display apparatus of the separate type shown in FIG. 32 requires a space between the liquid crystal panels and the projector optics for the installation of the two dichroic mirrors and, therefore, the back focus f.sub.B (i.e., the distance from the back vertex, or the vertex of the rearmost lens element of the projection lens assembly, to the back focal point) must have a considerably great value. On the other hand, in order for the projection display apparatus of the integrated type shown in FIG. 33 to be made compact, the distance over which the rays of light are projected from the light source must be as small as possible and, to accomplish this, the projection lens assembly must have a smaller focal length f and a great angle of projection, that is, a wide-angle lens should be used for the projection display apparatus. (It is to be noted that the term "angle of projection" referred to above may be interchangeable with a popular photographic term "angle of view" used in connection with a camera lens, but differs from the latter in terms of the direction of travel of rays of light).
For the projection lens assemblies used in the separate type, a projection lens assembly having a focal length f of 90 mm and a back focus f.sub.B of 160 mm has been realized, having a back focus ratio (f.sub.B /f) of 1.8. If an attempt is made to manufacture a compact projection display apparatus of an integrated type using the same liquid crystal panels and the same color combining optics as those used in the separate type, a rough calculation has indicated that the possible projection lens assembly should have a focal length f of 60 mm with the back focus ratio (f.sub.B /f) of 2.5 or preferably of a value greater than 2.7.
A retrofocus lens assembly wherein front and rear lens groups having negative and positive powers, respectively, disposed along a common optical axis in this order from the screen is known as a lens assembly having a back focus f.sub.B greater than the focal length f. A lens assembly having a back focus ratio f.sub.B /f of 2.7 and yet having a reduced distortion cannot be found in either the field of camera lenses or in any other field although what appears to be an exception is a fish-eye lens assembly. While the fish-eye lens assembly having a back focus ratio f.sub.B /f greater than 3 is available, a considerable distortion occurs in this fish-eye lens assembly and, therefore, where a liquid crystal display device employed in the projection display apparatus employs matrix electrodes, the use of this fish-eye lens assembly would result in a pin-cushion distortion of the projected color picture.
A variety of projection lens assemblies for use in a projection display apparatus utilizing light valves have been suggested in, for example, U.S. Pat. No. 5,042,929, issued Aug. 27, 1991, and No. 4,913,540, issued Apr. 3, 1990, and Japanese Laid-open Patent Publication No. 3-145613, published Jun. 20, 1991. However, all of those disclosed lens assemblies have a smaller back focus ratio f.sub.B /f and do not satisfy a requirement of the back focus ratio f.sub.B /f of 2.7.
In general, an increase in back focus f.sub.B and a reduction in focal length f are incompatible to each other and, therefore, the projection lens assembly having even a back focus ratio f.sub.B /f of 2.7 has been considered difficult to realize.